1. Technical Field
The present invention relates to systems and methods for the production of acids, such as aromatic carboxylic acids. More particularly, the present invention relates to the high shear production of aromatic carboxylic acids (e.g., terephthalic acid) which are sparingly soluble in acetic acid; the production of benzoic acid via gas/liquid phase partial oxidation of toluene; the production of methylbenzoic acid isomers and phthalic acid isomers from the corresponding xylene isomers; and more particularly to the acceleration of such reactions by high shear mixing.
2. Background of the Invention
Terephthalic acid is one of three isomeric organic phthalic compounds having the formula C6H4(COOH)2. Terephthalic acid is the para-form of phthalic acid, also known as 1,4-benzenecarboxylic acid, benzene-1,4-dicarboxylic acid, para-phthalic acid, TPA, and PTA. Terephthalic acid is a commodity chemical, used primarily as a precursor to the polyester polymers, primarily polyethylene terephthalate (PET), that are used to make clothing and bottles. Terephthalic acid has also been utilized for various purposes in addition to the fiber field and PET-bottle production, specifically for the production of PET-film and engineering plastics, as well as for a poultry feed additive. Phthalic acid derivatives are also routinely utilized in the production of dyes, medicine, synthetic perfumes, pesticides, and other chemical compounds.
Current state-of-the art technology for the manufacture of terephthalic acid involves the liquid phase oxidation of p-xylene feedstock using molecular oxygen or air in a solvent comprising lower C2 to C6 aliphatic mono-carboxylic acid, usually acetic acid, in the presence of a dissolved heavy metal (e.g. cobalt and/or manganese) catalyst system incorporating a promoter such as bromine. The reaction is carried out in at least one stirred vessel under elevated temperature and pressure conditions, typically 150° C. to 250° C. and 6 to 30 bars respectively, with air being sparged into the reaction mixture. The process typically produces terephthalic acid in high yield, e.g. at least 95%. Isothermal reaction conditions are generally maintained in the oxidation vessel by allowing evaporation of the solvent, together with water produced in the reaction. The resulting vapor is condensed and returned to the reaction vessel as reflux. Terephthalic acid is only sparingly soluble in the solvent and a substantial proportion of the product typically precipitates during the course of the reaction in conventional production methods. As a result of such precipitation, impurities such as 4-carboxybenzaldehyde (also known as 4-CBA, benzaldehyde-4-carboxylic acid, and terephthalaldehydic acid) and color bodies, including p-toluic acid, co-precipitate with the terephthalic acid, producing a crude product which generally must be purified to meet end-use requirements, e.g. requirements of polyester producers.
Purification typically comprises dissolving the oxidation product comprising crude terephthalic acid in water and, under elevated temperature and pressure, contacting the aqueous solution with hydrogen in the presence of a hydrogenation catalyst. The purified terephthalic acid is typically recovered thereafter by crystallization (cooling in a stepwise manner) and various solid-liquid separation techniques.
Accordingly, there is a need in the industry for enhanced systems and processes of producing aromatic carboxylic acids, such as terephthalic acid. Desirably, the system and method allow for minimal or substantially no purification being required downstream of the oxidation to meet purity requirements. Systems and methods for production of purified terephthalic acid without the need for extensive downstream purification and/or with reduced size (and thus cost) oxidation apparatus are desirable so as to more economically provide purified terephthalic acid.
Benzoic acid (carboxybenzene) is used to make a large number of chemicals. For example, benzoic acid is used to produce benzoyl chloride, by treatment of benzoic acid with thionyl chloride, phosgene or one of the chlorides of phosphorus. Benzoyl chloride is an important starting material for several benzoic acid derivates like benzyl benzoate, which is used for artificial flavors and insect repellents. Benzoyl peroxide is obtained by treatment of benzoic acid with peroxide. The peroxide is useful as a radical starter in polymerization reactions and also a component in cosmetic products. Benzoate plasticizers, such as the glycol-, diethylenegiveol- and triethyleneglycol esters are obtained by transesterification of methyl benzoate with the corresponding diol. Alternatively these species arise by treatment of benzoylchloride with the diol. These plasticizers are used similarly to those derived from terephthalic acid ester. Phenol is obtained by oxidative decarboxylation of benzoic acid at 300° C. to 400° C. Benzoic acid is also used as a food preservative, and as a constituent of ointments for the treatment of fungal skin diseases and acne.
The methylbenzoic acids are used in various industrial processes, including the production of chemicals, drugs, paints, and enamels. 4-Methylbenzoic acid (p-toluic acid) is a substituted benzoic acid that is used in the chemical industry to make terephthalic acid, which, in turn, is used industrially to produce polyethylene terephthalate (PET). PET is a thermoplastic polymer resin of the polyester family and is an important raw material used in synthetic fibers. It is also used in the manufacture of a wide variety of containers, in thermoforming applications, and in resins combined with glass fiber. 3-methylbenzoic acid (m-toluic acid) is used industrially as a precursor in the production of the insecticide DEET (N,N diethyl-m-toluamide), among other uses. 2-methylbenzoic acid (o-toluic acid) is widely used as a raw material for agricultural chemicals, medicines and polymerization initiators.
Benzoic acid and the 2-, 3-, and 4-methylbenzoic acid isomers are typically produced by partially oxidizing toluene or o-, m-, and p-xylene, respectively, with oxygen or air. On an industrial scale, both of the methyl groups in p-xylene are oxidized by oxygen or air to produce terephthalic acid (benzene-1,4-dicarboxylic acid or p-phthalic acid). Such processes are strongly influenced by a number of factors, such as temperatures, pressures, and the nature of the catalyst used, if any. Appropriate selection of these factors is important, as selection influences the reaction trend, the reaction velocity, and the overall technical and economic balance of the production, both in terms of yield and catalyst consumption, and also from the point of view of the intricacy and costs of installation and upkeep. These costs are influenced, for example, by the pressures attained, the consumption of thermal energy for reaching desired temperatures, and the intricacy and the number of component parts of the installation. For instance, in many applications it is desirable to enhance the degree of conversion of toluene or xylene. While increasing the reaction pressure may increase reaction rate, it also increases wear of the materials constituting the reactors, the piping, and the mechanical parts of the plant, as well as any ancillary devices. Most existing processes and production facilities for making benzoic, methylbenzoic acids, and phthalic acids are subject to a variety of constraints such as product yield, plant size, energy consumption and mass flow limitations. Accordingly, there is continuing interest in improving the ways that benzoic acid, methylbenzoic acid isomers, and phthalic acid isomers are produced.